WFU Physics Ph. D. Thesis Defense

TITLE: Measuring the Microscale Mechanical Properties of Fibrin Fibers and Cancer Cells

SPEAKER: Justin Sigley,

TIME: Monday November 11, 2013 at 11 AM

PLACE: Room 103 Olin Physical Laboratory

The microscale material properties dictate the macroscale behavior of biological systems. Fibrinogen, one of the most abundant proteins in the blood, is converted into fibrin fibers that perform the essential mechanical task of stemming the flow of blood. Fibrinogen fibers can be fabricated by a technique called electrospinning. We studied the mechanical properties of dry, electrospun fibrinogen fibers using a combined atomic force/fluorescence microscopy technique. The mechanical properties of these electrospun fibers is important due to their potential use in tissue engineering and their biocompatibility. The same atomic force/fluorescence microscopy technique is used to measure the mechanical properties of fibrin fibers formed from patient plasma. The mechanical properties of blood clots have been related to diseases such as cardiovascular disease and diabetes, but the mechanisms responsible for their mechanical properties are not well understood. The glycation of fibrinogen, a marker for glycemic control in diabetic patients, did not affect the mechanical properties of individual fibrin fibers. The modulus of the fibers was found to be directly related to the diameter of the fibers and provides evidence for a non-uniform density of protofibrils within the fiber.

Cancerous and non-cancerous cells have different mechanical properties arising from biochemical alterations as normal cells transform to cancer cells. This transformation may affect the mobility of proteins and small molecules within the cell. We used a novel technique called Raster Image Correlation Spectroscopy (RICS) to measure the diffusion coefficients of fluorescent proteins in living cells. We found the diffusion coefficients of these proteins are not affected by neoplastic transformation in the cytoplasm, but the mobility of the fluorescent proteins is altered in the nucleus of the cells. This suggests neoplastic transformation alters the intra-nuclear structure on a length scale similar to the sizes of the proteins measured. We cross-validated the RICS results with Fluorescence Recovery After Photobleaching (FRAP) experiments. The RICS and FRAP results agree in 87% of the measurements. We then demonstrate the accuracy of RICS measurements by performing a RICS analysis on quantum dots undergoing a programmatically controlled 2D random walk. Direct confirmation of RICS results provides progress toward a gold standard for molecular dynamics measurements in live cells.